record a mac address of a networkcard

[Joachim Schiele <js@dune2.de>]

is there a possibility to record the mac address of the networkcards
i need this because i don't want the clients being able to spoof the proxy 
requests to the firewall. i want them to be the only coputer who is able to 
make requests to the firewall

pool(20 computers) -> proxy(linux) -> firewall(firewall) -> cisco-router -> 
isdn(net)

the proxy and the firewall have their own networkcards

firewall
eth0
eth1

proxy
eth0
eth1

tnx for any advice :))))

joachim